British actress Keira Knightley has reportedly stripped off to pose naked for an acclaimed artist.The Pirates of the Caribbean actress, who bared all to film a bath scene with Sienna Miller in their 2008 movie The Edge of Love, has allegedly been shedding her clothes during sessions at London's Halcyon Gallery for Mitch Griffiths. A source tells The Mail on Sunday, "It’s considered a great honour to be painted by Mitch. He is a highly respected nude painter and is painting a number of subjects for an exhibition next year. He is delighted Keira agreed to take part." Last year body-proud Knightley demanded that promotional adverts for her 2008 movie The Duchess were left untouched after the promos for 2004's King Arthur were changed to enhance her breasts.